P-type semiconductor composed of magnesium, silicon, tin, and germanium, and method for manufacturing the same

ABSTRACT

A manufacturing method for a p-type semiconductor formed by sintering a compound represented by the general chemical formula: Mg 2 Si X Sn Y Ge Z  (where X+Y+Z=1, X&gt;0, and Y&gt;0, Z&gt;0). The p-type semiconductor has a composition in which X is in the range of 0.00&lt;X≦0.25, and Z satisfies the relationship: −1.00X+0.40≧Z≧−2.00X+0.10, where Z&gt;0.00, and Y is in the range of 0.60≦Y≦0.95, and Z satisfies either of the relationships: −1.00Y+1.00≧Z≧−1.00Y+0.75, where 0.60≦Y≦0.90 and Z&gt;0.00, and −2.00Y+1.90≧Z≧−1.00Y+0.75, where 0.90≦Y≦0.95 and Z&gt;0.00.

BACKGROUND

The present disclosure relates to a p-type semiconductor composed ofmagnesium, silicon, tin, and germanium, and a method for manufacturingthe same.

Recently, attempts have been made to improve thermoelectric performanceby reducing the resistivity by carrier concentration control by doping aMg₂Si-based material with a p-type dopant (for example, Ag, Ga, or Li).Examples of such materials include:

Mg₂Si+1 at % Ag, ZT=0.1 (560 K): (See M. Akasaka et al., J. Appl. Phys.,104, 013703, 2008).

Mg₂Si_(0.6)Ge_(0.4)+0.8% Ga, ZT=0.36 (625 K): (see H. Lhou-Mouko et al.,J. Alloys Compd., 509, pp. 6503-6508, 2011).

Mg₂Si_(0.25)Sn_(0.75)+Ag-20000 ppm and Li-5000 ppm, ZT=0.32 (600 K):(see Japanese Published Unexamined Patent Application No. 2010-37641).

SUMMARY

Mg₂(SiSn) and Mg₂(SiGe) have been studied as promising p-typesemiconductors, however, there are no known attempts that have beendeveloped into semiconductors on a practical level. P-typesemiconductors Mg₂(SiSn) and Mg₂(SiGe) are solid solutions with Mg₂Si,and it is believed that Ge and Sn contribute to p-type conduction in thesolid solutions. Therefore, elements that can change the Si site of thebase composition must form an anti-fluorite structure with Mg. Suchmetal elements are limited to silicon (Si), germanium (Ge), tin (Sn),and lead (Pb) of Group 14. However, Pb is generally excluded from thislist of elements because it is a hazardous metal.

An attempt was made to improve the performance of a p-typethermoelectric semiconductor by using the following quaternary system:

Mg₂Si_(X)Sn_(Y)Ge_(Z), where X+Y+Z=1 and X>0, Y>0, Z>0.

When using ternary Mg₂SiSn, only two phase diagrams of Mg₂Si and Mg₂Snare considered. However, when using the above-mentioned quaternarysystem, four phase diagrams of Mg₂Ge, Mg₂(SiSn), Mg₂(SiGe), andMg₂(SnGe) must be considered. As a result, preparation of a single-phasesample of the quaternary system is difficult. These are problems thatthe present disclosure is intended to solve.

In view of the circumstances described above, the present disclosureaddresses the above-described problems. One embodiment according to thepresent disclosure provides a method for manufacturing a p-typesemiconductor composed of magnesium, silicon, tin, and germanium. Themethod of manufacturing the p-type semiconductor involves sintering acompound represented by the following general chemical formula:

Mg₂Si_(X)Sn_(Y)Ge_(Z), where X+Y+Z=1 and X>0, Y>0, Z>0 and is obtainedthrough liquid-solid reaction of magnesium, silicon, tin, and germaniumas raw materials. The obtained semiconductor is a p-type semiconductorsatisfying the following equations:

X is in the range of 0.00<X≦0.25, and Z satisfies the relationship of−1.00X+0.40≧Z≧−2.00X+0.10, where Z>0.00, and

Y is in the range of 0.60≦Y≦0.95, and Z satisfies either of thefollowing relationships:

−1.00Y+1.00≧Z≧−1.00Y+0.75, when 0.60≦Y≦0.90 and Z>0.00, and

−2.00Y+1.90≧Z≧−1.00Y+0.75, when 0.90≦Y≦0.95 and Z>0.00.

Another embodiment according to the present disclosure provides a p-typesemiconductor composed of magnesium, silicon, tin, and germanium. Thep-type semiconductor is manufactured by sintering a material representedby the following general chemical formula:

Mg₂Si_(X)Sn_(Y)Ge_(Z), where X+Y+Z=1 and X>0, Y>0, Z>0 obtained throughliquid-solid reaction of magnesium, silicon, tin, and germanium as rawmaterials.

The obtained semiconductor is a p-type semiconductor satisfying thefollowing equations:

X is in the range of 0.00<X≦0.25, and Z satisfies the relationship:

−1.00X+0.40≧Z≧−2.00X+0.10, where Z>0.00, and

Y is in the range of 0.60≦Y≦0.95, and Z satisfies either of thefollowing relationships:

−1.00Y+1.00≧Z≧−1.00Y+0.75, when 0.60≦Y≦0.90 and Z>0.00, and

−2.00Y+1.90≧Z≧−1.00Y+0.75, when 0.90≦Y≦0.95 and Z>0.00.

The above embodiments make it possible to easily manufacture a p-typesemiconductor represented by the following general chemical formula:

Mg₂Si_(X)Sn_(Y)Ge_(Z), where X+Y+Z=1 and X>0, Y>0, Z>0.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process chart for obtaining a p-type semiconductor accordingto an embodiment of the present disclosure.

FIG. 2A is a table showing compositions of weighed values of p-typesemiconductors, and FIG. 2B is a table showing compositions of p-typesemiconductors according to the present disclosure.

FIG. 3 is a graph showing X-ray diffraction measurement results ofMg₂Si_(0.25)Sn_(Y)Ge_(Z) in various p-type semiconductors.

FIG. 4 is a table showing the compositions (weighed values) ofMg₂Si_(X)Sn_(Y)Ge_(Z) and thermoelectric properties thereof at a roomtemperature in various p-type semiconductors.

FIG. 5A, FIG. 5B, and FIG. 5C are graphs showing the relationshipsbetween the Ge composition and the Seebeck coefficient α, the thermalconductivity κ, and the resistivity ρ in various p-type semiconductors.

FIG. 6 is a graph showing the relationship between X and Z in variousp-type semiconductors.

FIG. 7 is a graph showing the relationship between Y and Z in variousp-type semiconductors.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a p-type semiconductor made of asintered compact of an intermetallic compound of magnesium (Mg), silicon(Si), tin (Sn), and germanium (Ge), which is represented by thefollowing general chemical formula:

Mg₂Si_(X)Sn_(Y)Ge_(Z), wherein X+Y+Z=1 and X>0, Y>0, Z>0. The sinteredcompact of the intermetallic compound is manufactured as follows.

Granular Mg and Sn with a grain size of approximately 2 to 10 mm areprepared, and powdery Si and Ge with a grain size of approximatelyseveral tens of μm are prepared. Predetermined amounts of thesematerials are weighed and put into a carbon board. The carbon board iscovered with a carbon lid, and heated for 4 hours at an absolutetemperature of 1173 K under an atmosphere of 0.1 MPa ArH₂ (3 weight %hydrogen) to cause a liquid-solid reaction.

The obtained solid solution is pulverized into powder with a grain sizeof 38 to 75 μm, and sintered by hot-pressing. The sintering pressure isstandardized to 50 MPa and the sintering time is standardized to 1 hour.The sintering temperature was determined according to each Sncomposition amount Y. The sintering temperature is set to 1190 K whenY=0, 1040 K when Y=0.60 or 0.65, and 930 K when Y=0.75 or 0.90.

Weighed values (mole ratios) and compositions (mole ratios) of severalsintered compacts obtained as described above are shown in the tables ofFIG. 2. According to this, the weighed values (FIG. 2A) and thecompositions (FIG. 2B) of the sintered compacts are found to havechanged little when comparing the amounts of the Mg, Si, Sn and Gebefore and after sintering.

Further, in FIG. 3, results of X-ray diffraction measurement ofMg₂Si_(0.25)Sn_(Y)Ge_(Z) obtained as described above are shown.According to X-ray diffraction, peaks were observed with all of thesintered compacts existing between Mg₂Si and Mg₂Sn having ananti-fluorite structure. Only peaks caused by the anti-fluoritestructure were observed, and no peaks were observed with oxides, Mg₂Si,Mg₂Ge, and Mg₂Sn. Based on the data in FIG. 3, it was confirmed that allof the sintered compacts were single-phase. The same results wereobtained with other sintered compacts.

Next, the conduction types, the Seebeck coefficients α (μV/K), thethermal conductivities κ (W/mK), and the resistivities ρ (Ωm) of varioussintered compacts of Mg₂Si_(X)Sn_(Y)Ge_(Z) thus obtained are shown inthe table of FIG. 4. In FIG. 5, graphs showing the relationships betweenthe Ge composition and the Seebeck coefficient α, the thermalconductivity κ, and the resistivity ρ are shown.

Next, conduction types of semiconductors with variable values for X andZ based on the results of FIG. 4 are plotted in FIG. 6. Conduction typesof these semiconductors in which the values between Y and Z change areplotted in FIG. 7. From these graphs, the conduction type border betweenthe p-type and the n-type is found to have changed linearly. In eachgraph, ∘ indicates p-type, and x indicates n-type.

First, observing the relationship between X and Z in FIG. 6, as a p-typesemiconductor, X is in the range of 0.00<X≦0.25. When X is in thisrange, a maximum value Z_(max) and a minimum value Z_(min) of Z forobtaining a p-type semiconductor change linearly in relation to X. Alinear function of Z_(max) and a linear function of Z_(min) arerespectively obtained as follows:

Z_(max)=−1.00X+0.40

Z_(min)=−2.00X+0.10, where Z_(min)>0.00.

It is confirmed that, as a p-type semiconductor, X and Z fall within theshaded range shown in FIG. 6, that is, X and Z satisfy the followingrelationship:

−1.00X+0.40≧Z≧−2.00X+0.10, where Z>0.00.

Observing the relationship between Y and Z in FIG. 7, as a p-typesemiconductor, Y is in the range of 0.60≦Y≦0.95. When Y is in thisrange, a maximum value Z_(max) and a minimum value Z_(min) of Z forobtaining a p-type semiconductor change linearly in relation to Y, and alinear function of Z_(max) and a linear function of Z_(min) are obtainedas follows:

Z_(max)=−1.00Y+1.00, where 0.60≦Y≦0.90

Z_(max)=−2.00Y+1.90, where 0.90≦Y≦0.95

Z_(min)=−1.00Y+0.75, where Z_(min)>0.00.

It is confirmed that as a p-type semiconductor, Y and Z fall within theshaded range shown in FIG. 7, that is, Y and Z satisfy the followingrelationship:

−1.00Y+1.00≧Z≧−1.00Y+0.75, where 0.60≦Y≦0.90 and Z>0.00, or

−2.00Y+1.90≧Z≧−1.00Y+0.75, where 0.90≦Y≦0.95 and Z>0.00.

The present disclosure is applicable to obtaining of a p-typesemiconductor composed of Mg₂Si_(X)Sn_(Y)Ge_(Z).

The invention claimed is:
 1. A method for manufacturing a p-typesemiconductor composed of magnesium, silicon, tin, and germaniumcomprising: mixing magnesium, silicon, tin, and germanium as rawmaterials, obtaining, by liquid-solid reaction, a solid solution of themagnesium, silicon, tin, and germanium mixture represented by thefollowing general chemical formula: Mg₂Si_(X)Sn_(Y)Ge_(Z), where X+Y+Z=1and X>0, Y>0, Z>0, and sintering the obtained mixture to produce ap-type semiconductor, the p-type semiconductor represented by thefollowing general chemical formula: Mg₂Si_(X)Sn_(Y)Ge_(Z), wherein: X isin the range of 0.00<X≦0.25, and Z satisfies the relationship of−1.00X+0.40≧Z≧−2.00X+0.10, and Z>0.00, and Y is in the range of0.60≦Y≦0.95, and Z satisfies either of the following relationships:−1.00Y+1.00≧Z≧−1.00Y+0.75, where 0.60≦Y≦0.90 and Z>0.00, and−2.00Y+1.90≧Z≧−1.00Y+0.75, where 0.90≦Y≦0.95 and Z>0.00.
 2. The methodaccording to claim 1, wherein Y is in the range of 0.65≦Y≦0.90.
 3. Ap-type semiconductor composed of magnesium, silicon, tin, and germanium,wherein: the p-type semiconductor is manufactured by a liquid-solidreaction of magnesium, silicon, tin, and germanium as raw materials toobtain a material represented by the following general chemical formula:Mg₂Si_(X)Sn_(Y)Ge_(Z), where X+Y+Z=1 and X>0, Y>0, Z>0, followed bysintering to obtain a the p-type semiconductor, the p-type semiconductorrepresented by the following general chemical formula:Mg₂Si_(X)Sn_(Y)Ge_(Z), where: X is in the range of 0.00<X≦0.25, and Zsatisfies the relationship: −1.00X+0.40≧Z≧−2.00X+0.10, where Z>0.00, andY is in the range of 0.60≦Y≦0.95, and Z satisfies either of thefollowing relationships: −1.00Y+1.00≧Z≧−1.00Y+0.75, where 0.60≦Y≦0.90and Z>0.00, and −2.00Y+1.90≧Z≧−1.00Y+0.75, where 0.90≦Y≦0.95 and Z>0.00.4. The p-type semiconductor of claim 3, wherein Y is in the range of0.65≦Y≦0.90.